On Marien Street, a Hamburg thoroughfare lined with apartment buildings with red-tiled roofs, No. 54 is an unremarkable off-white house.
In a country that is home to about 7 million immigrants, the sight of several clean-cut Arab students coming and going from the lace-curtained home is not unusual.
This was the home of Mohamed Atta, the presumed ringleader of the Sept. 11 attacks who apparently piloted one of the jets into the World Trade Center. Atta shared the home with Marwan al-Shehhi, suspected of crashing the second plane in New York, and Ziad Jarrah, suspected of flying the plane that crashed in Pennsylvania.
In hindsight, authorities are learning lessons about how they could have detected the presence of a terrorist cell. But one of the biggest needs, they say, is for better cooperation among agencies - cooperation that may have linked Atta's cell with operatives now under arrest in Madrid.
Atta was believed to have been in contact with Imad Eddin Barakat Yarbas, also know as Abu Dahdah, head of a Spanish Islamic group that called itself "The Soldiers of Allah." Mr. Dahdah reportedly traveled widely, contacting other Islamic extremists in Indonesia, Malaysia, and Jordan.
The Hamburg cell's cooperation with the one in Madrid shows how easy it is for Al Qaeda groups to blend into countries with large immigrant populations. "We had no suspicions that there was an Al Qaeda cell in Hamburg," a high-ranking German secret service official says. "We knew there were some Islamic extremists in the city, but these young men were totally unsuspicious. They didn't so much as get a parking ticket."
German and American law enforcement agencies are also searching for three fugitives: Said Bahaji of Germany, Zakariya Essabar of Morocco, and Ramzi bin al-Shibh of Yemen.
Analysts speculate that, although militants involved in the Sept. 11 attacks lived in many European countries, they were particularly attracted to Germany because of its lax immigration laws and population of 3 million Muslims.
Hamburg, in particular, has a large, well-integrated Muslim community, including the largest Afghan community in Europe. Because Hamburg's Muslim community was quiet and moderate, it was also not under heavy surveillance.
Out of 122 secret service agents in Hamburg, only one was assigned to "Islamic extremists," which include everything from the ethnic-Albanian UCK to the Turkish PKK. Until Sept. 11, Hamburg security forces were more concerned with the city's left-wing extremists who periodically engage in destructive demonstrations.
After the end of the cold war, German secret services were cut back and the department which dealt specifically with the Middle East was closed in the late 1990s. "When the East-West conflict was over, everyone thought we would face fewer risks," says Kai Hirschmann of the German Federal College for Security Studies. "In the '90s no one ever thought Germany could be a base or a target for international terrorism. That was seriously underestimated. The Hamburg authorities were very unprepared," Mr. Hirschmann says. "The federal police have very few people who speak the pertinent languages or can be sent to infiltrate these organizations and that is really the only way to know what their plans are."
The German secret service did lock onto Bahaji, a local student for a short period in 1998 because he spent a lot of time at a local mosque with Syrian businessman Marmoun Darkazanli.
Mr. Darkazanli had opened a bank account in Hamburg with Marndouh Mahmud Salim, an alleged financier of Osama bin Laden's Al Qaeda network.
Had German intelligence followed Bahaji more closely, they might have discovered that he had texts by bin Laden in his apartment. But even this clue would have told agents little.
German security officials say more inter-agency and international cooperation may have helped them catch the Hamburg cell. Atta had three different identities in Germany but because the numerous security bureaucracies in Germany share few records, the inconsistency went unnoticed.
"Until Sept. 11, US authorities were very hesitant to contribute their intelligence into the international security discussion," says Wolfgang Dicke, general secretary of the German Police Union. "They expected European intelligence agencies to spill everything."
Recently, a string of allegations have surfaced concerning lack of cooperation between intelligence services. German investigators in Hamburg complain that FBI agents withheld intelligence from their German colleagues in an attempt to get to witnesses first, while American officials in Belgium say they were refused information by the Belgian authorities.
Now, emphasis has shifted to what could be done better. "There is no golden key to prevent terrorism in the future. Insurance doesn't exist," the German secret service official concludes. "The only real option is for intelligence services all over the world to communicate more."